Amino acids
Cambridge A-Level Chemistry (9701) · Unit 34: Organic nitrogen compounds · 8 flashcards
Amino acids is topic 34.4 in the Cambridge A-Level Chemistry (9701) syllabus , positioned in Unit 34 — Organic nitrogen compounds , alongside Primary and secondary amines, Phenylamine and azo compounds and Amides. In one line: A zwitterion is an amino acid molecule with both a positive and negative electrical charge. This occurs when the amino group is protonated (NH₃⁺) and the carboxyl group is deprotonated (COO⁻).
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 8 flashcards — 3 definitions and 5 key concepts — covering the precise wording mark schemes reward. Use the 3 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
Zwitterion
A zwitterion is an amino acid molecule with both a positive and negative electrical charge. This occurs when the amino group is protonated (NH₃⁺) and the carboxyl group is deprotonated (COO⁻).
What the Cambridge 9701 syllabus says
Official 2025-2027 spec · A2 LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- describe the acid / base properties of amino acids and the formation of zwitterions, to include the isoelectric point
- describe the formation of amide (peptide) bonds between amino acids to give di- and tripeptides
- interpret and predict the results of electrophoresis on mixtures of amino acids and dipeptides at varying pHs (the assembling of the apparatus will not be tested)
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9701 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Amino acids
- › At a pH higher than the isoelectric point, the zwitterion loses a proton to form a negatively charged ion.
- › Remember that alkyl groups are electron-donating; this concentrates negative charge on the oxygen, making the ethoxide ion a stronger base.
- › Classify reaction products accurately; condensation with 2,4-DNPH produces an organic orange precipitate, whereas dichromate reduction produces inorganic green Cr3+ ions.
- › Include H2O and H+ on the left-hand side and NH4+ on the right when writing balanced equations for acid hydrolysis of amides.
- › Identify all gaseous products, including water vapour, to determine the true mass of the remaining solid residue.
What is a zwitterion?
A zwitterion is an amino acid molecule with both a positive and negative electrical charge. This occurs when the amino group is protonated (NH₃⁺) and the carboxyl group is deprotonated (COO⁻).
Define the isoelectric point (pI) of an amino acid.
The isoelectric point (pI) is the pH at which a particular amino acid exists predominantly as a zwitterion. At this pH, the amino acid has no net electrical charge and will not migrate in an electric field.
Describe how amino acids behave in acidic conditions (pH < pI).
In acidic conditions (pH < pI), amino acids act as bases and accept protons. The amino group (NH₂) becomes protonated (NH₃⁺), and the amino acid carries a net positive charge (+1).
Describe how amino acids behave in alkaline conditions (pH > pI).
In alkaline conditions (pH > pI), amino acids act as acids and lose protons. The carboxyl group (COOH) becomes deprotonated (COO⁻), and the amino acid carries a net negative charge (-1).
Describe the formation of a peptide bond.
A peptide bond (amide bond) is formed via a condensation reaction between the carboxyl group of one amino acid and the amino group of another. This reaction releases a water molecule (H₂O).
What is a dipeptide?
A dipeptide is a molecule consisting of two amino acids joined by a single peptide bond.
What is electrophoresis and how can it be used to separate amino acids?
Electrophoresis is a technique that separates charged molecules based on their size and charge by applying an electric field. Amino acids migrate towards the electrode with the opposite charge, with the separation dependent on their pI and the buffer pH.
Predict the direction of movement of an amino acid during electrophoresis if the pH of the buffer is less than the amino acid's pI.
If the pH < pI, the amino acid will be positively charged overall. Therefore, it will migrate towards the cathode (negative electrode).
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Amino acids sits alongside these A-Level Chemistry decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Amino acids deck
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